Edge caching using HTTP headers

ABSTRACT

Disclosed is a program for pre-fetching resources. A computer, communicatively coupled to a plurality of client computers and a server computer, identifies a resource, through an examination of one or more HTTP server logs, that is cached on at least one of the plurality of client computers and has been validated by the server computer. The computer determines to pre-fetch the resource based on one or more predefined rules, at least one of the predefined rules including a threshold number of responses validating the resource that must be received by the computer. The computer pre-fetches and caches the resource from the server computer. The computer receives a request for the resource from a client computer that does not have the resource cached. The computer validates resource cached locally and sends the resource to the client computer from the local cache on the computer.

FIELD OF THE INVENTION

The present invention relates generally to the field of data transferover a computer network, and more particularly to a technique for cachepre-fetching.

BACKGROUND OF THE INVENTION

The Hypertext Transfer Protocol (HTTP) is an application protocol fordistributed information systems and is the foundation of datacommunication for the World Wide Web. Caching HTTP objects (html files,video, audio, images, etc.) is a well known technique to improveperformance of applications requesting the objects. The HTTP/1.1protocol (and later) includes a number of elements intended to makecaching work efficiently. The goal of caching is to reduce, whereverpossible, the need to send requests, and when a request must be sent, toreduce the need to send full responses in reply. Reducing the need tosend requests lowers the number of network round-trips required, and istypically accomplished through “expiration” mechanisms allowing therequesting application or computer to assume that a cached copy of aresource is current if within a specified validity period. Reducing theneed to send full responses lowers the strain on network bandwidth, andis typically accomplished through “validation” mechanisms. When a cachehas a stale entry that it would like to use as a response to anapplication's or client's request, it first has to check with the originserver (or possibly an intermediate cache with a fresh response) to seeif the cached (stale) entry is still usable. This is known as“validating” the cache entry. Known methods of validation include theuse of “last-modified dates” and “entity tag” (ETag) values included inHTTP headers.

Intermediate “caching” servers that can store resources locally so thatsubsequent requests for the resource can be returned directly from theintermediate server often have the capability to pre-fetch a resource inanticipation of the resource being requested by a client computer.

SUMMARY

Embodiments of the present invention disclose a method, computer programproduct, and computer system for pre-fetching content for anintermediate cache. A computer, communicatively coupled to a pluralityof client computers and a server computer, identifies at least oneresource, through an examination of one or more HTTP server logs, thatis cached on at least one of the plurality of client computers and hasbeen validated by the server computer. The computer determines topre-fetch the at least one resource based on one or more predefinedrules, at least one of the predefined rules including a threshold numberof responses validating the at least one resource that must be receivedby the computer. The computer pre-fetches the at least one resource fromthe server computer. The computer caches the at least one resourcelocally. The computer receives a request for the at least one resourcefrom a client computer, of the plurality of client computers, that doesnot have the at least one resource cached on the client computer. Thecomputer validates the at least one resource cached locally, and sendsthe at least one resource to the client computer from the local cache onthe computer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram of a distributed data processing environmentcomprising multiple client computers connected to multiple servercomputers through a network with at least one edge caching server, inaccordance with an embodiment of the present invention.

FIG. 2 is a flowchart of the steps of an HTTP header analyzer program onedge caching server within the data processing environment of FIG. 1, inaccordance with an embodiment of the present invention.

FIG. 3 depicts an exemplary process flow for validating a resource wherean HTTP caching directive has been overwritten.

FIG. 4 depicts a block diagram of internal and external components ofthe edge caching server.

DETAILED DESCRIPTION

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system, method or computer programproduct. Accordingly, aspects of the present invention may take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module” or “system.”Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer-readablemedium(s) having computer readable program code/instructions embodiedthereon.

Any combination of one or more computer-readable medium(s) may beutilized. The computer-readable medium may be a computer-readable signalmedium or a computer-readable storage medium. A computer-readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, or device, or any suitable combination of the foregoing. Morespecific examples (a non-exhaustive list) of the computer-readablestorage medium would include the following: an electrical connectionhaving one or more wires, a portable computer diskette, a hard disk, arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM or Flash memory), an optical fiber,a portable compact disc read-only memory (CD-ROM), an optical storagedevice, a magnetic storage device, or any suitable combination of theforegoing. In the context of this document, a computer-readable storagemedium may be any tangible medium that can contain, or store a programfor use by or in connection with an instruction execution system,apparatus, or device. A computer-readable tangible storage device, asused herein, is not to be construed as being transitory signals per se,such as radio waves or other freely propagating electromagnetic waves,electromagnetic waves propagating through a waveguide or othertransmission media (e.g., light pulses passing through a fiber-opticcable), or electrical signals transmitted through a wire.

A computer-readable signal medium may include a propagated data signalwith computer-readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Acomputer-readable signal medium may be any computer-readable medium thatis not a computer-readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device.

Program code embodied on a computer-readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for aspects of thepresent invention may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java®, Smalltalk, C++or the like and conventional proceduralprogramming languages, such as the “C” programming language or similarprogramming languages. The program code may execute entirely on a user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).The terms “Java” and “Smalltalk” may be subject to trademark rights invarious jurisdictions throughout the world and are used here only inreference to the products or services properly denominated by the marksto the extent that such trademark rights may exist.

Aspects of the present invention are described below with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in acomputer-readable medium that can direct a computer, other programmabledata processing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer-readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce acomputer-implemented process such that the instructions which execute onthe computer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

The present invention will now be described in detail with reference tothe Figures. FIG. 1 illustrates a distributed data processingenvironment, generally designated 100, according to one embodiment ofthe present invention.

In the illustrated embodiment, distributed data processing environment100 depicts an exemplary networked environment where various clientdevices (client computers 102, 104 and 106) can access content from oneor more content providers (server computers 108, 110, and 112) through aseries of networked connections. Requests from any of client computers102, 104, and 106 are relayed through any number of intermediatecomputers, including edge caching server 114, which, in one embodiment,provides forwarding of the requests (via network 116) to the appropriateserver computer through DNS mapping. The initial connection to edgecaching server 114 can be provided through a standard network of wiredand/or wireless components or, in another embodiment, through cellulartransmission towers, as is common when a client device is a smart phone.In a scenario where the client devices communicate initially through acellular tower, edge caching server 114 may be the nearest servercomputer to a radio network controller (RNC) handling transmissions frommultiple towers.

Edge caching refers to the use of caching servers (intermediate or proxycomputers) to store resources/content closer to end users. For example,if a client computer (an end user) accesses an origin server (e.g., acontent server computer hosting a web site and/or providing othercontent) and downloads some static content that gets cached, eachsubsequent user (being routed through the caching server) will getserved the content directly from the caching server until the contentexpires. The effectiveness of a caching server depends on the amount ofdata or content that can be served from its cache, achieved by a highrate of “cache hits.”

Edge caching server 114 may be any programmable electronic devicecapable of caching content and of routing content between any of theclient computers 102, 104, or 106 and any of the server computers 108,110, or 112. In one embodiment, edge caching server 114 represents acomputing system utilizing clustered computers and components to act asa single pool of seamless resources when accessed through a network.

Various techniques exist for determining what to cache and what not tocache. As a preliminary matter, caching servers typically store allallowable resources (e.g., resources where a cache-control directive inthe HTTP header is “public”). Caching servers may also “pre-fetch”resources by requesting and receiving content before the content isrequested by a client computer. Pre-fetching can be done based on ananalysis of HTTP server logs, popularity of the content in contentdelivery networks (CDNs) or the Internet. Just as cache-controldirectives in an HTTP header can indicate that content may be cached,such directives can also indicate that content may not be cached. Forexample, while a “public” cache-control directive indicates that theresponse may be cached/stored by any cache, a “private” cache-controldirective indicates that all or part of the response message is intendedfor a single user and must not be cached by a shared cache. This allowsan origin server to state that the specified parts of the response areintended for only one user and are not a valid response for requests byother users. A private or non-shared cache (e.g., the client computer'scache) may cache the response. A “no-cache” cache-control directiveallows caching by all computers, but forces caches (both of proxycomputers and browsers/clients) to submit a request for validationbefore releasing a cached copy. HTTP provides a number of additionalcache-control directive headers.

Embodiments of the present invention recognize that, especially ininstances where a communication channel of a client computer has limitedbandwidth (e.g., a smart phone or computer connecting to the Internetthrough a cellular tower connected to the core network using a radiolink), HTTP directives that prevent caching can greatly impactperformance of the network—especially when applications request highvolumes of data (e.g., video streaming, etc.) Often content providerswant to prevent network/edge caching to increase page hits at the originserver, which in turn increases advertising revenue. In an exemplaryoperation, client computer 106, depicted in FIG. 1 as a smart phone,makes an HTTP GET request for content such as one or more of html files,audio, images, or video from server computer 112. The request is routedthrough edge caching server 114. Server computer 112 returns the contentwith the HTTP header “Cache-control: private,” directing edge cachingserver 114 to not store the content in its local cache, cache storage118. Edge caching server 114 forwards the content to client computer106. Client computer 106, as a private end-user computer, may store thecontent locally. If client computer 106 sends a subsequent conditionalrequest with an “if-modified-since” or other validation header, thesubsequent request cannot be validated by edge caching server 114 andthe request is routed all the way back to server computer 112, which mayrespond with either an updated copy of the content or a “not-modified”HTTP header, confirming the freshness of the content stored in the localcache of client computer 106. Any time a different client computer,e.g., client computer 102 or 104, requests the content, because thecontent was not cached at edge caching server 114, or any otherintermediate server, the content must be retrieved from server computer112.

HTTP header analyzer program 120, residing on edge caching computer 114,stores, or has access to, HTTP server logs 122 that track HTTP headersreceived from both client computer 106 and server computer 112. Innormal operation, any analysis of HTTP logs focuses on new requests andrequests where any cached responses may be used (e.g., HTTP message type200 response objects) for caching and pre-fetching. Embodiments of thepresent invention, additionally or alternatively, use HTTP headeranalyzer program 120 to track validation requests and/or validationresponses to determine content that should be pre-fetched and cached. Avalidation request is any conditional request asking for a fresh copy ofstored content if the content has been modified since it was originallycached. Typically, a conditional request contains an “if-modified-since”header. A validation response indicates that the content has not beenmodified and, typically, such a response contains an HTTP message type304 “not-modified” response header. Analyzing validationrequests/responses can be especially useful because consistentvalidation requests may be a strong indicator that content will bepopular, and numerous “not-modified” responses indicate that the contenttends to remain static, making it a good candidate for caching as thepotential for the content to change is low.

Based, in part, on the validation requests/responses, HTTP headeranalyzer program 120 determines which content should be pre-fetched. Inembodiments of the present invention, depending on the determinedcontent, HTTP header analyzer program 120 may also have to set rules toallow caching where HTTP header directives seek to prevent it. HTTPheader analyzer program 120 uses pre-fetch agent 124 to collect thedetermined resources. HTTP header analyzer program 120 may use storedcaching policy and rules definitions 126 in conjunction with trackedvalidation requests/responses to determine the content to pre-fetch, andadditionally may use stored caching policy and rules definitions 126 tostore its own determined caching policies.

FIG. 2 depicts the operational steps of HTTP header analyzer program 120for determining resources to pre-fetch, in accordance with oneembodiment of the present invention.

HTTP header analyzer program 120 analyzes HTTP server logs 122 todetermine content most commonly seeking validation (step 202). HTTPserver logs 122 can, in one embodiment, be HTTP header logs trackingheaders of all HTTP requests and responses, and HTTP header analyzerprogram 120 can search the headers to identify conditional requests forcontent (e.g., “if-modified-since” requests) and “not-modified”responses. In another embodiment, HTTP server logs 122 are separatelycreated logs in which edge caching server 114 can separately storeconditional requests for content and “not-modified” responses, therebyavoiding the necessity to parse out the validation requests/responses.Additionally, in a hierarchical caching network, the peer cachingservers (server computers at the same level of the hierarchy) canexchange information. Thus, edge caching server 114 can, in oneembodiment, gather and analyze HTTP server logs from all peer cachingservers to obtain the most accurate information for determining whichcontent to pre-fetch. If a new edge caching server is installed, it cansimilarly query its peer caching servers for their HTTP logs topre-populate (through pre-fetching) the empty cache of the new edgecaching server. This can be referred to as “warming up” the cache.

After determining the pertinent content, HTTP header analyzer program120 applies predefined caching policy and rules definitions 126 toproduce a list of resources to be pre-fetched (step 204). One exemplaryrule might state that any resource where validation has been requestedmore than a specified number of times, within a specified time period,and having a size greater than a specified threshold should bepre-fetched. Other exemplary rules might require that validationrequests have been received from at least a specified number of clientcomputers or that a predefined threshold of “not-modified” responseshave been returned from the origin server computer. Such rules mayindicate that the exact same content is being returned to multipleclient computers and thus the prevention of caching is inefficient.Other rules might also limit the included resources to a top percentageof the resources meeting the criteria.

HTTP header analyzer program 120 moves to the first resource in the list(step 206) and determines whether a header override will be required(decision 208). For example, if responses to requests for the resourceare received with “public” or “no-cache” headers, no special treatmentwill be required during a subsequent request for the resource. These arecacheable responses. However, responses having “private” headersindicate that the requested content should not be stored in edge cachingserver 114 and caching the response would require an override of thecaching policy.

If a header override is required, HTTP header analyzer program 120 setsan override rule (step 210). The override rule indicates that a responseto a given request may be cached. In one embodiment HTTP header analyzerprogram 120 sets an internal flag associated with the request for theresource to indicate that the resource is cacheable.

HTTP header analyzer program 120 requests the resource (step 212) andcaches the resource in local cache storage (step 214). If a header ofthe received resource indicates that the content is not cacheable, HTTPheader analyzer program 120 checks a request of the received resourceand compares the request to any flagged requests to verify that theresource may be cached. A person of ordinary skill in the art willrecognize that determining if a header override is required and settingthe override rule may be implemented after the resource is received andbefore the resource is cached. Pre-fetch agent 124 may perform theactual process of forwarding the request to server computer 112 andreceiving the response.

HTTP header analyzer program 120 determines whether there are moreresources in the list (decision 216), and if so, moves to the nextresource on the list (step 218) to perform the same process.

Subsequent requests for a pre-fetched resource can now be handled byedge caching server 114 according to standard operation protocolsallowing the cached resource to be served to the requesting clientcomputer directly from edge caching server 114. However, in a preferredembodiment, where a requested resource is cached on edge caching server114 by virtue of a rule or policy overriding an HTTP caching directive,edge caching server 114 validates the cached resource prior to sendingit to the client computer. In such a manner, the server computer maycontinue to get “page hits.” An exemplary process flow is illustrated inFIG. 3.

In FIG. 3, edge caching server 114 receives a request for a resourcefrom a client computer (step 302). If the request received is aconditional request, this indicates that the requesting client computerhas its own cached copy and wants to validate the cached copy or receivea current copy of the resource. Normal operation relays the conditionalrequest to server computer 112 and relays the response from servercomputer 112 back to the requesting client computer. No deviation fromstandard protocols is necessary. If the request is not conditional (therequesting computer must receive the resource) and the resources is notcached locally on edge server computer 114, again, the request isrelayed to server computer 112 and the resource is in turn relayed backthe requesting client computer. No deviation from standard protocol isnecessary. If the request is not conditional, and the resource is cachedlocally on edge caching server 114, but the resource is cacheablewithout any special overriding policies or rules, edge caching server114 treats the request and cached resource normally and can send theresource directly to the requesting client computer or validate theresource according to standard HTTP protocols. However, if edge servercomputer 114 determines that the request is not conditional (step 304),and edge server computer 114 determines that the resource is cachedlocally (step 306), and edge server computer 114 determines thatresource is not typically cacheable on edge server computer 114 (step308), edge server computer 114 validates the resource (step 310) withserver computer 112. This ensures that server computer 112 remains awareof all requests for resources not typically cacheable on edge servercomputer 114. Subsequent to validating the resource, edge servercomputer 114 sends the resource from the local cache to the requestingclient computer (step 312).

FIG. 4 depicts a block diagram of components of edge caching server 114in accordance with an illustrative embodiment. It should be appreciatedthat FIG. 4 provides only an illustration of one implementation and doesnot imply any limitations with regard to the environment in whichdifferent embodiments may be implemented. Many modifications to thedepicted environment may be made.

Edge caching server 114 includes communications fabric 402, whichprovides communications between computer processor(s) 404, memory 406,persistent storage 408, communications unit 410, and input/output (I/O)interface(s) 412.

Memory 406 and persistent storage 408 are examples of computer-readabletangible storage devices and media. Memory 406 may be, for example, oneor more random access memories (RAM) 414, cache memory 416, or any othersuitable volatile or non-volatile storage device.

HTTP header analyzer program 120 and pre-fetch agent 124 are stored inpersistent storage 408 for execution by one or more of the respectivecomputer processors 404 via one or more memories of memory 406. In theembodiment illustrated in FIG. 4, persistent storage 408 includes flashmemory. Alternatively, or in addition to flash memory, persistentstorage 408 may include a magnetic disk storage device of an internalhard drive, a solid state drive, a semiconductor storage device,read-only memory (ROM), EPROM, or any other computer-readable tangiblestorage device that is capable of storing program instructions ordigital information.

The media used by persistent storage 408 may also be removable. Forexample, a removable hard drive may be used for persistent storage 408.Other examples include an optical or magnetic disk that is inserted intoa drive for transfer onto another storage device that is also a part ofpersistent storage 408, or other removable storage devices such as athumb drive or smart card.

Communications unit 410, in these examples, provides for communicationswith other computers and devices. In these examples, communications unit410 includes one or more network interface cards. Communications unit410 may provide communications through the use of either or bothphysical and wireless communications links. HTTP header analyzer program120 and pre-fetch agent 124 may be downloaded to persistent storage 408through communications unit 410.

I/O interface(s) 412 allows for input and output of data with otherdevices that may be connected to edge caching server 114. For example,I/O interface 412 may provide a connection to external devices 418 suchas a keyboard, keypad, a touch screen, and/or some other suitable inputdevice. I/O interface(s) 412 may also connect to a display 420.

Display 420 provides a mechanism to display data to a user and may be,for example, a computer monitor.

The programs described herein are identified based upon the applicationfor which they are implemented in a specific embodiment of theinvention. However, it should be appreciated that any particular programnomenclature herein is used merely for convenience, and thus theinvention should not be limited to use solely in any specificapplication identified and/or implied by such nomenclature.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

What is claimed is:
 1. A computer program product for pre-fetchingcontent, the computer program product comprising: one or morecomputer-readable tangible storage devices and program instructions,stored on the one or more computer-readable tangible storage devices,which when executed; identify at least one resource, through anexamination of one or more HTTP server logs, that is cached on at leastone of a plurality of client computers and has been validated by aserver computer; determine to pre-fetch the at least one resource basedon one or more predefined rules, at least one of the predefined rulesincluding a threshold number of responses validating the at least oneresource; pre-fetch the at least one resource from the server computer;cache the at least one resource locally; receive a request for the atleast one resource from a client computer, of the plurality of clientcomputers, that does not have the at least one resource cached on theclient computer; and send the at least one resource to the clientcomputer from the local cache; and prior to caching the at least oneresource locally: responsive to determining that an HTTP header includesa cache-control directive that dictates that the at least one resourceshould not be cached, set a flag associated with the at least oneresource to indicate that the at least one resource is locallycacheable.
 2. The computer program product of claim 1, wherein: theprogram instructions to determine that the HTTP header that dictatesthat the at least one resource should not be cached comprises programinstructions to determine that the HTTP header is a “private”cache-control directive.
 3. The computer program product of claim 1,further comprising program instructions, stored on the one or morecomputer-readable tangible storage devices, which when executed: priorto sending the at least one resource to the client computer from thelocal cache: determine that the at least one resource cached locally isnot cacheable absent a rule or policy overriding an HTTP cachingdirective; and validate the at least one resource.
 4. The computerprogram product of claim 1, wherein the one or more predefined rulesfurther comprise one or more of: a threshold number of client computersthat have cached the at least one resource; a threshold number of clientcomputers that have requested validation of the at least one resource; athreshold number of validation requests received; and a threshold sizeof the at least one resource.
 5. The computer program product of claim3, wherein the program instructions, which when executed, validate theat least one resource comprise program instructions, which whenexecuted: send a conditional request to the server computer such that ifthe at least one resource on the server computer has been modified, theserver computer sends a current version of the at least one resource,and if the at least one resource has not been modified, the servercomputer sends a response indicating that the at least one resource iscurrent.
 6. The computer program product of claim 1, wherein the atleast one resource is a video.
 7. The computer program product of claim1, wherein the program instructions, which when executed, identify theat least one resource comprise program instructions, which whenexecuted: search the one or more HTTP server logs for“if-modified-since” request headers and “not-modified” response headers.8. The computer program product of claim 1, further comprising programinstructions, stored on the one or more computer-readable tangiblestorage devices, which when executed, query at least one peer cachingcomputer in a hierarchical caching network for at least one of the oneor more HTTP server logs.
 9. A computer system for pre-fetching content,the computer system comprising: one or more computer processors; one ormore computer-readable tangible storage devices; program instructions,stored on the one or more computer-readable tangible storage devices forexecution by at least one of the one or more computer processors, whichwhen executed by the at least of the one or more computer processors:identify at least one resource, through an examination of one or moreHTTP server logs, that is cached on at least one of a plurality ofclient computers and has been validated by a server computer; determineto pre-fetch the at least one resource based on one or more predefinedrules, at least one of the predefined rules including a threshold numberof responses validating the at least one resource; pre-fetch the atleast one resource from the server computer; cache the at least oneresource locally; receive a request for the at least one resource from aclient computer, of the plurality of client computers, that does nothave the at least one resource cached on the client computer; and sendthe at least one resource to the client computer from the local cache;and prior to caching the at least one resource locally: responsive todetermining that an HTTP header includes a cache-control directive thatdictates that the at least one resource should not be cached, set a flagassociated with the at least one resource to indicate that the at leastone resource is locally cacheable.
 10. The computer system of claim 9,wherein: the program instructions to determine that the HTTP header thatdictates that the at least one resource should not be cached comprisesprogram instructions to determine that the HTTP header is a “private”cache-control directive.
 11. The computer system of claim 9, furthercomprising program instructions, stored on the one or morecomputer-readable tangible storage devices for execution by at least oneof the one or more computer processors, which when executed by the atleast one of the one or more computer processors: prior to sending theat least one resource to the client computer from the local cache:determine that the at least one resource cached locally is not cacheableabsent a rule or policy overriding an HTTP caching directive; andvalidate the at least one resource.
 12. The computer system of claim 9,wherein the one or more predefined rules further comprise one or moreof: a threshold number of client computers that have cached the at leastone resource; a threshold number of client computers that have requestedvalidation of the at least one resource; a threshold number ofvalidation requests received; and a threshold size of the at least oneresource.
 13. The computer system of claim 11, wherein the programinstructions, which when executed, validate the at least one resource,comprise program instructions, which when executed: send a conditionalrequest to the server computer such that if the at least one resource onthe server computer has been modified, the server computer sends acurrent version of the at least one resource, and if the at least oneresource has not been modified, the server computer sends a responseindicating that the at least one resource is current.
 14. The computersystem of claim 9, wherein the at least one resource is a video.
 15. Thecomputer system of claim 9, wherein the program instructions, which whenexecuted, identify the at least one resource, comprise programinstructions, which when executed: search the one or more HTTP serverlogs for “if-modified-since” request headers and “not-modified” responseheaders.
 16. The computer system of claim 9, further comprising programinstructions, stored on the one or more computer-readable tangiblestorage devices for execution by at least one of the one or morecomputer processors, which when executed by the at least one of the oneor more computer processors, query at least one peer caching computer ina hierarchical caching network for at least one of the one or more HTTPserver logs.